Permanent-magnet synchronous motors are brushless motors characterized by low cost, physical ruggedness, and simple construction. There are essentially three types of construction for such motors. The "surface-magnet" type has radially magnetized arc-shaped magnets attached to the surface of a smooth rotor and is widely available with either sinusoidal (due to distributed phase windings) or trapezoidal (due to concentrated phase windings) back-EMF voltage characteristics. The "interior-magnet" type has alternately poled, radially or circumferentially magnetized, rectangular magnets embedded in a smooth rotor, with distributed or concentrated phase windings. The "hybrid-stepper" type has a single axially magnetized cylindrical rotor magnet enclosed by a two-piece rotor shell having projecting rotor teeth (with teeth offset between the two pieces), and the stator has concentrated phase windings on pairs of projecting poles. In the absence of magnetic saturation, the surface-magnet and hybrid-stepper types generally do not exhibit angle-dependent phase inductance, whereas the interior magnet type generally does exhibit this characteristic.
Regardless of their specific construction features, permanent-magnet synchronous motors are attractive as servo drives because of their high power densities. The stator phases of such motors may be electrically excited to produce a controlled torque on the rotor, the torque being proportional to the field intensity of the rotor magnet(s) and the amplitude of the stator phase excitation, thus permitting control of the rotor motion. When such motors are employed for high performance servo drive applications which require precise control of rotor motion, the use of feedback signals representing rotor position and rotor velocity becomes necessary. The most common method for obtaining sufficiently accurate feedback signals is to mount a high-resolution magnetic resolver or optical encoder to the rotor shaft, in order to directly measure the rotor position with sufficient accuracy, and then to electronically process this direct rotor position measurement to obtain an indirect measurement of rotor velocity. There are several disadvantages associated with shaft-mounted rotor position sensors, including their cost, size, mass, and potential unreliability.
Another approach for obtaining the feedback signals needed to control permanent-magnet synchronous motors is to install Hall-effect position sensing devices inside the stator housing. Such Hall-effect devices detect only the polarity of the rotor's magnetic field and, hence, provide only a coarsely quantized measure of rotor position. In general, the quantization factor (i.e., resolution) associated with Hall-effect devices is determined by the number of rotor poles and stator phases, and cannot be improved upon for a given motor. By contrast, shaft-mounted position sensors have quantization factors that depend only on the precision of the sensor construction, not on the construction details of the motor to which it is attached. The coarse quantization of the rotor position signal obtained from Hall-effect devices also leads to difficulties in the determination of the corresponding rotor velocity signal. At low velocity, the rotor passes from one quantization interval to another infrequently and, hence, the indications of rotor velocity cannot be updated at a sufficiently high rate to be accurate.
A motor equipped with a shaft-mounted high accuracy position sensor can be controlled in servo fashion, with the stator phase excitation continuously modulated substantially in response to the accurately measured values of rotor position and rotor velocity. Due to the high accuracy of the feedback signals in such a servo system, it is possible to control the instantaneous value of rotor torque and thus to achieve precise control of rotor motion. The motion control goals that can be achieved in such a servo system include velocity control, in which the rotor is commanded to regulate to a fixed desired velocity or to track a time-trajectory of desired velocities, and position control, in which the rotor is commanded to regulate to a fixed desired position or to track a time-trajectory of desired positions. Since the phase excitation is determined by the feedback signals, the accuracy of the motion control critically depends on the accuracy of the sensor measurements. If Hall-effect devices are used as the only means of measuring rotor position and rotor velocity, the quantization of the feedback signals limits the possible control actions to selection of commutation instants or phase firing angles. Since phase excitation is not continously modulated using Hall-effect devices, only the average value (rather than instantaneous value) of rotor torque can be controlled, and consequently the accuracy of rotor motion is rather limited.
As disclosed in prior art, permanent-magnet synchronous motors can be used simultaneously as actuators and sensors of motion. For high performance servo drive applications, this combined actuator sensor mode of operation requires, at minimum, the highly accurate estimation of rotor position from purely electrical measurements taken at the stator terminals (with highly accurate estimation of rotor velocity achieved by processing the rotor position estimates in the traditional way). Alternatively, rotor position and rotor velocity may be estimated simultaneously from stator terminal measurements, instead of using the traditional sequential processing. Measurable stator terminal signals are limited to the phase currents (the currents flowing through the phase windings), and either the applied phase voltages (in case the phase is receiving excitation from the power source and hence has a nonzero current flowing through its winding) or the open-circuit phase voltages (in case the phase is unexcited and hence is disconnected from the power source with no current flowing through its winding).
It is well known that if the rotor of a permanent-magnet synchronous motor is rotating with significant velocity, then the rotating magnetic field set up by the rotor magnet(s) will induce an electromotive force, or back-EMF voltage, on the stator phase windings. The back-EMF voltage is dependent upon both rotor position and velocity and, when it is present, it influences the stator phase dynamics. Consequently, the back-EMF voltage, when it is present, can potentially play a useful role in rotor motion estimation schemes. However, the back-EMF voltage is periodic with respect to rotor position (with an integer number of cycles per revolution determined by the construction of the rotor) and, more significantly, is linearly proportional to the rotor velocity. Hence, the back-EMF voltage is not present on any stator phase if the rotor is not rotating. If the rotor is rotating but with negligible velocity, then the back-EMF voltage will be contaminated by noise. The back-EMF voltage therefore possesses no direct utility for the estimation of rotor position when the rotor is completely or practically motionless. Even when the back-EMF voltage is present, it cannot be directly measured at the stator terminals of a given stator phase unless this same stator phase is unexcited (open-circuited with no current flowing through it). Due to the periodicity of the back-EMF voltage with respect to rotor position, schemes using this signal cannot estimate the rotor position in an absolute sense (at least without including some heuristic procedures), but instead can only estimate rotor position relative to the electrical cycle.
Subject to the limitations discussed above, the back-EMF voltage has been used in prior art to estimate rotor motion (i.e., rotor position and rotor velocity). Other rotor motion estimation schemes, such as those relying on naturally present variable phase inductance or saturation-induced variable phase inductance, are not suitable for all types of permanent-magnet synchronous motors. Most of the existing rotor motion estimation schemes based on the back-EMF voltage do not have the objective of estimating rotor position with accuracies typical of traditional shaft mounted sensors, such as magnetic resolvers or optical encoders. Instead, the goal of most existing rotor motion estimation schemes is simply to eliminate the need for Hall-effect position sensing devices mounted inside the stator housing. Since quantization can be tolerated in this case, these rotor motion estimation schemes seek to detect events, measurable at the stator terminals, which are expected to occur once each step within the commutation sequence. For example, a detectable event directly related to the back-EMF voltage is the zero-crossing of an open-circuit phase voltage. All so-called event detection methods for estimating rotor motion have the disadvantage of coarse feedback quantization, as well as the corresponding disadvantage of limited motion control accuracy. Thus, there is a general need for a more precise method of estimating rotor motion, not limited by the quantization effects of event detection schemes.
Prior art also discloses rotor motion estimation schemes with the potential to serve as replacements for traditional high accuracy shaft mounted sensors, subject to certain limitations. For example, U.S. Pat. No. 5,134,349 to Kruse discloses a sensorless controller for permanent-magnet synchronous motors which continuously modulates the phase excitation to achieve instantaneous torque control, with the feedback signals obtained by processing the back-EMF voltage in a continuous fashion. However, this technique is disclosed only for motors with two stator phases (or motors with three interconnected stator phases), with sinusoidal back-EMF voltage characteristics, and with sensing coils mounted inside the stator. The most appropriate motors for many applications have more than two or three phases, and have non-sinusoidal back-EMF voltage characteristics. Moreover, the use of internal sensing coils is a disadvantage because such coils add to the cost and size of the motor, reduce the power density of the motor, and decrease reliability due to the additional wiring connections between the motor and the control electronics. Therefore, a need still exists for a rotor motion estimation method for motors with any number of stator phases, with any periodic back-EMF voltage shape, and without sensing coils mounted inside the stator.
A more elaborate method for potentially replacing high accuracy shaft mounted sensors is disclosed in the article "Real-Time Observer-Based (Adaptive) Control of a Permanent-Magnet Synchronous Motor Without Mechanical Sensors," by R. B. Sepe and J. H. Lang, 1991. A sensorless controller is described which continuously modulates the phase excitation to achieve instantaneous torque control, with feedback signals obtained by simulating (solving forward in time from assumed initial conditions) a mathematical model of the motor and its load, augmented with a correction term used to compensate for errors between the values of stator current predicted by the simulated model and the measured values of stator current. However, this technique is disclosed only for motors with two stator phases (or motors with three interconnected stator phases), with sinusoidal back-EMF voltage characteristics, and with a known model for the rotor load. The disadvantages associated with the required number of phases and required back-EMF voltage shape have already been set forth. Furthermore, rotor load parameters such as friction coefficients, load torque, and load inertia often are difficult or impossible to measure or approximate accurately, and consequently this method of estimation is adversely affected by such unavoidable parametric errors. There still exists a need for a rotor motion estimation method for motors with any number of stator phases, with any periodic back-EMF voltage shape, and which does not need explicit knowledge of rotor load parameters.
Prior art rotor motion estimation methods fail to function as desired at low velocities and at zero velocity. In the method of Kruse, for example, the loss of the back-EMF voltage at low and zero velocities requires an abrupt transition to a hold-mode wherein large currents are applied to the phases in order to hold the rotor in place. Also, in the method of Sepe and Lang, the error between the estimated and actual rotor positions does not converge to zero at a reasonable rate at low velocities, and may actually diverge at low velocities for a motor wherein the phase inductance is independent of rotor position. Selection of gains for the correction term is not systematic. Hence, there still exists a need for a technique which is capable of estimating rotor position at standstill. It is to the provision of this need and the additional needs identified above that the present invention is primarily directed.